This invention relates to improvements in so-called dynamic loudspeakers wherein a coil is provided with electrical current and is disposed in a magnetic field to produce an axial driving force on the coil and on a sound radiator attached to the coil.
Most conventional loudspeakers in use today comprise a cylindrical voice coil that is disposed in a radial magnetic field created by the pole pieces of a magnet. The pole pieces are arranged to provide a circular gap, and the coil, having one or more layers of turns of wire, is disposed in the gap. The ends of the coil are connected to a source of signal current, such as an amplifier. The current passing through the length of the coil creates a field that links with the magnetic field of the magnet to produce a driving force on the coil. The force (F) on the coil is equal to BLI, where B is the flux density, L is the length of wire immersed in the magnetic flux, and I is the current.
In many types of speaker drives, the length of the cylindrical coil is fabricated to be approximately equal to the fixed thickness or depth of the magnetic poles at the gap. In this manner, substantially the entire useful length of the coil is immersed in the magnetic field, enhancing total efficiency.
The production of low frequency sound presents special problems for a woofer having a moving coil drive. In order to produce low frequency sound of sufficient intensity, it is desirable for the available peak to peak excursion of the voice coil be as large as possible, in order for the sound diaphragm to excite large volumes of air. Such large excursions, however, cause the voice coil to move either partially or entirely out of the region of constant magnetic flux, producing distortions, since the force per unit current decreases as the active length of the voice coil is reduced. The term "active length" is defined herein as only that part of the coil which remains in the magnetic field in the gap.
In order to overcome the low frequency distortion problem and increase available excursion, longer or overhanging voice coils have been employed, in which the length of the coil is greater than the thickness of the magnetic gap. The use of a longer coil, however, results in a substantial loss of efficiency, since only the active portion of the coil in the magnetic field contributes to the driving force. The use of a longer coil also creates greater total resistance to the flow of current and places a limit on power handling capacity due to the production of heat, which is proportional to the resistance.
From the foregoing, it may be seen that if large peak to peak excursions are required in a movable coil drive for a speaker, problems are encountered that result in loss of linearity, loss of efficiency, or both, particularly in lowermost frequencies of human hearing, i.e., below 75 Hz. Thus, it would be desirable to devise a moving-coil drive having features that would overcome or avoid these problems.